1. Field of the Invention
The present invention relates to a diffractive optical element used as a lens of an optical system.
2. Description of the Related Art
In a diffractive optical element used as a lens of an optical system, it is known that two diffractive gratings are closely disposed and a material and a grating height of each diffractive grating are appropriately set to obtain a high diffraction efficiency in a wide wavelength range. When light enters the diffractive optical element including a grating surface and a grating wall surface, the incident light is reflected or refracted by the grating wall surface to generate unnecessary light (flare). Japanese Patent Laid-Open No. 2003-315526 discloses a diffractive optical element which is provided with a covering layer that covers a grating wall surface by an absorber or a metal material to suppress the generation of a wave having a spherical wave shape to improve a diffraction efficiency of a designed diffractive order. Japanese Patent Laid-Open No. 2005-115176 discloses a diffractive optical element which has a metal material formed on a grating wall surface and light of designed incident light entering the grating wall surface is reflected in a direction of a diffractive angle to improve a diffraction efficiency of the designed diffractive order. Japanese Patent Laid-Open No. 2009-217139 discloses a calculation of a diffraction efficiency using the rigorous coupled wave analysis (RCWA).
In the diffractive optical element used as the lens of the optical system, the unnecessary light which is especially a problem is unnecessary light caused by a total reflection that is generated at an interface of a high refractive index medium and a low refractive index medium by light incident at an oblique incident angle (an off-screen light incident angle) that is different from that of the designed incident angle. However, Japanese Patent Laid-Open No. 2003-315526 and Japanese Patent Laid-Open No. 2005-115176 do not disclose the problem that the unnecessary light is generated by the light incident at the oblique incident angle (the off-screen light incident angle) that is different from that of the designed incident light. Therefore, in the configuration of the diffractive optical element that is disclosed in Japanese Patent Laid-Open No. 2003-315526 and Japanese Patent Laid-Open No. 2005-115176, the unnecessary light cannot be reduced and the deterioration of the image performance cannot be efficiently decreased.